Refurbishing an AVO 2 Panel Tester

[chriswood1900]

Some time ago I purchased a fairly tatty and not used for a long time Navy or Admiralty pattern tester. The case was very crusty with most of the joints coming apart and the plywood top and bottom de-laminating. The tester had been protected in the case and but it did not look safe to use as the mains wiring and umbilical cord had perished so badly there were bare wires showing.

I started with overhauling the box dismantling it and gluing it back together with numerous clamps and soaked the de-laminating plywood in diluted PVA glue which would flow into the wood and clamped that as well. I then rubbed it down and primed it with grey paint the original colour seemed to be BS 381C – Light Admiralty Grey 697 so I purchased a can of that and sprayed it to get a good finish. On the original case the screws and fittings were visible through the paint so I left it the same. I bought some Cheney Catches and a leather strap to replace the original and cleaned up all the hinges screws and fittings I wanted to retain, I was pleased with the result.

Then on to refurbishing the tester itself I knew there were several challenges as it had a broken plug on the umbilical and the cover on the pin selector was broken in several places so I had collected a couple of other challenged testers at BVWS auctions for a small amount of money to provide spares.
The basic Tester was OK cosmetically so I opened it up and replaced the mains lead with 3 core to earth the case and made up a new umbilical with silicone wire in a sheathing using the plug from another tester. All the caps were replaced in the 2 units. Inside the tester one of the 2 metal rectifiers was loose in the case probably from being dropped. So I replaced them with IN4005 diodes and heat shrank some sheathing to cover the bare wires. Leaving MR1 in place as it supported R3.

I then started some tests which showed things were not working properly. I was fortunate that with one the broken testers came some copies of the original setup procedures (attached below) so I worked through those. I made myself a couple of test leads with wander plugs to 4mm so I could plug leads directly into the 9 pin socket to check voltages. I decided to use an AVO 7 as I guessed that was what AVO would have used when writing the instructions. The heater anode, screen and heater voltages were within spec. but the anode selector switching was very unreliable. A quick look showed the contacts were out of line with the switch position. This was followed by a major battle with the grub screw which held the knob on. It seems that the grubs crews used were prone to seize as most of them had, so they were left for many hours with silicone based releasing agent. Most of them then undid but not the Anode selector so I set about finding and alternative to breaking the knob. The brass shank can be removed through the case by undoing the Bakelite contact holder inside the tester and withdraw the shaft with the knob on it. I tried again to undo the grub screw but only succeeded in breaking my screwdriver bit so I used the shaft from another tester and now anode selection worked and matched the pinouts at the output socket excepting D1 and D2 were reversed. After further examination and tracing the pins onto the valve panel I realised the original AVO instructions had an error reversing D1 and D2. So we were ready to go on with some tests.
With the sensitivity control fully clockwise it sort of worked but the minute it was moved the needle went off the scale. I went round the houses several times looking at posts on similar problems until after about 2 hours thinking through it hit me, I’d had a grey moment!! and put the diode replacing MR1 in backwards, reversed it and everything started working again properly. A Few final tweaks to the calibration and I was getting sensible readings with my calibration valves.
The final problem hinged around the cut-out which is only on the navy versions, it was making poor contact and it did not seem to work properly, I could not find any setup details so I came up with my own. First cleaning the contacts with some thin card soaked in contact cleaner I then adjusted screw A so the contacts fully opened then adjusted screw B so that the spring had just enough tension to hold the contacts open the contacts normally being held closed by the residual magnetism until excess current is drawn. It is reset by a thin wire attached to the underside of the MA/V G.INS switch. If you see the last photo I have marked the 2 screws.

Overall it was successful with a nice looking unit, I had spent more than was economic but so what I had added to my small selection of military style testers.
I hope this post proves useful to others and I attach below some pictures and the information sheets I used.
Chris

[chriswood1900]

The AVO Second to Fourth Echelon setup instructions. Lovely name!

[The Philpott]

Nice job. I don't know what that impregnated/woven insulation material actually is, but it certainly seems to survive well closeted away from UV light. Have seen it in yellow. red and mainly black variations. It seems to react slightly with heat and copper to produce a green deposit wherever there are solder joints, but having said that i haven't seen a case where this has made a single core conductor rot all the way through.

Dave

[Kevin Hoyland]

Hello Chris.
The case looks outstanding and thanks for the write- up. I am working on the same tester but my umbilcal cables have all disintegrated all 9 of them so I replaced them. I have replaced all the capacitors and the fitted 2 new diodes but my tester is still not working. The resistors are not two far out.I will keep at it.

Regards
Kevin.

[chriswood1900]

Hi Kevin
Have you followed the setup procedure attached to post 2 measured at the 9 pin socket on the tester unit, if all the AC voltages are correct, check the grid voltages which changes when you move the MA/V switch, you will need a 2 channel scope triggered from the anode voltage and you will see it move to the opposite phase. Finally you are left with the balancing circuit and meter which should respond to the sensitivity control without any valve going negative as you turn it anticlockwise.
Good Luck
Chris

[chriswood1900]

Dave
Thanks for the kind words. Yes there were plenty of greenish deposits on all the solder joints but it did not seem to effect soldering at all and the wires stay clean. Everything in the Navy/ Admiralty seems to be covered in a varnish which I did not see in the standard version..
Regards
Chris

[Freya]

Hi Chris,

That's come up really nice, can you post a picture of the scale please.

[chriswood1900]

Stephen I will post a few more pictures but it will be a couple of days as I'm away working.
Chris

[Bobdger]

Hi Chris
Thanks for the witeup on your refurbising, The case looks wellgood. Please can you supply with the dimensions and more pics of your case as I would like to make one for my tester.
Thanks.
Bob

[chriswood1900]

Freya, herewith a scan of the scale.
Bob here is a photo of the box opened out and dimensions added. The box is mainly constructed from a mahogany or iroko finished from 12mm x 95mm pieces jointed at the corners with 6mm comb joints. The top and bottom are made from 9.5mm ply screwed on with 3/4" no 6 brass screws.
Outside dimensions are 339 x 294 x 210mm. Good luck with building it.
Regards
Chris

[Freya]

Hi Chris,

I was curious when you posted the original pictures as the scale did not appear to have the Broad arrow or the cut out light.
Here`s a few pics of mine which I believe is 1947 from the scale, yours would appear to be 1948.

[Bobdger]

Hi Chris
Thank you for the info.
Bob

[chriswood1900]

Stephen
I have two of these Admiralty tester one with a hole with red film which is where the scan came from and the one just rebuilt where the light is offset to the side and shines a red light across the scale. When you said you wanted a picture I thought you needed a copy of the scale so I scanned the scale from the faulty unit, here is the scale from the other one serial number 89841-956 which looks an original fitment!
Sorry if I've caused any confusion.
Chris